Large diesel engines, to be efficient, are operated at a constant speed. When operating at a constant speed, such an engine provides very high torque output to the output shaft of an associated transmission while such shaft is rotating at a slow speed. Consequently, such engines require large transmissions particularly at low output speeds. One method of providing a high torque output at such low speeds has been to use a recycling hydrostatic mechanical transmission of the design described by Orshansky in U.S. Pat. No. 3,580,107, issued May 25, 1971. Such a recycling transmission as attributed to Orshansky reduces the need for an excessively large hydrostatic unit at the high torque low output speed encountered in the starting range of a vehicle using a large diesel engine. A similar transmission atrributable to Hans Reichenbaecher is found in U.S. Pat. No. 3,106,855 issued Oct. 15, 1953. Control of both these systems is through a series of cams and push rods.
Hyrdostatic mechanical transmissions utilizing a recycling planetary group in the past have encountered difficulty in sensing the synchronous speed necessary in coupling the various range gear sets and holding this synchronous speed until the next range is coupled. As a corollary to this problem, a problem has been encountered in determining the most advantageous time to reverse the stroking of the hydrostatic units, whose displacement and speeds are inversely related, through the multiple and repetitive steps. Also encountered in the coupling engagement and disengagement, is the problem of energy waste when the operator strokes the hydrostatic units beyond the point of range synchronization.
As noted above, earlier transmissions of this type have required large hydrostatic units, especially in the starting range, thereby raising the cost of manufacture of the hydrostatic unit and the vehicle itself. Provision of a reverse range for heavy duty hydrostatic mechanical transmissions has proved to be a complex and costly problem. Most vexing has been the problem of controlling heavy duty transmissions in all vehicle speed requirements while running at near or full engine throttle. As a corollary to this problem, it is necessary to provide a safety device for neutralizing the transmission control when the vehicle is out of operation with the engine stopped. Finally, previous transmissions have failed to satisfactorily provide a constant output torque throughout the starting range without a complex structure.